pcatterall
Well-Known Member
My annode to prop circuit is via the engine I checked it as the annode was not depleted after 2years. The resistance reading was 700 ohms. I have no idea what it should be pleas help!
Resistance between annode and prop
- Thread starter pcatterall
- Start date
Billjratt
Well-Known Member
As near zero as possible. Try "star" connection rather than 'via' the engine or anywhere else.
VicS
Well-Known Member
Re: Resistance between anode and prop
Ideally zero!
"star" connection ?????
Ideally zero!
"star" connection ?????
Last edited:
Billjratt
Well-Known Member
Re: Resistance between anode and prop
Taking all the braids directly to the anode bolt rather than a single wire to various points in turn - cuts down on possible poor joints at one point isolating downline bits.
Taking all the braids directly to the anode bolt rather than a single wire to various points in turn - cuts down on possible poor joints at one point isolating downline bits.
VicS
Well-Known Member
Re: Resistance between anode and prop
but when he said "via" the engine he presumably meant simply a wire* from the anode to convenient a bolt on the engine, (and the flexible shaft coupling bridged), as a means of making a connection to the prop. You cannot go directly from the prop to the anode
*4mm2 PVC Insulated Multi-Stranded Single Core Copper Cable or larger is recommended by MGDuff
but when he said "via" the engine he presumably meant simply a wire* from the anode to convenient a bolt on the engine, (and the flexible shaft coupling bridged), as a means of making a connection to the prop. You cannot go directly from the prop to the anode
*4mm2 PVC Insulated Multi-Stranded Single Core Copper Cable or larger is recommended by MGDuff
pcatterall
Well-Known Member
Thanks all. I appreciate as near to zero as possible, I just don't understand how significant my reading was. It's just a few ohms direct from annode to prop.
Graham_Wright
Well-Known Member
Re: Resistance between anode and prop
Without being pedantic, you can if the anode is on the prop shaft.
Isn't that the best location especially if you subscribe to the view that "connecting things" should be avoided?
Without being pedantic, you can if the anode is on the prop shaft.
Isn't that the best location especially if you subscribe to the view that "connecting things" should be avoided?
VicS
Well-Known Member
I think you need to check through bit by bit.
Anode to engine block. Block to shaft. Across the coupling, Shaft to prop.
There's a good chance of a poor connection from engine to shaft because it relies on contact through bearings etc which will be full of oil. If you find a problem such as this that cannot easily be overcome you may want to fit an "Electro eliminator" or similar brush system making contact with the shaft inboard of the stern gland.
Alternatively you might consider that cathodic protection of the prop is not necessary if there is no evidence of recent corrosion. Or if it is can you fit a shaft anode or a prop nut anode
pcatterall
Well-Known Member
Thanks again, thanks Vic's. Yes exactly! I guess that the route via the engine and box is creating resistance and I guess that I could fit one of those contact gadgets! I have considered just connecting a lead while boat is not in use! BUT I would really like to know if the resistance I quoted really is too much and that I HAVE to take action...,..... The visually evidence suggests that the circuit is not efficient I just hoped that someone would have a handle on the figures!!!
Thanks again.
Thanks again.
VicS
Well-Known Member
I dont recall seeing any figures for maximum resistance quoted. but check it all out. You might find something causing the problem that can be fixed
I can see a shaft anode is not an option. You could use the temporary connection idea but it will be a PITA especially you forget it one day.
Birdseye
Well-Known Member
The rate at which an anode depletes depends on the current flow and the higher the resistance, the smaller the current. I have a problem of corrosion on my prop and a resistance of just 3 ohms. According to MG Duff even 3 ohms is too much and the condition of my prop confirms that
What sort of engine do you have? The Volvo MD series insulate the engine body from the prop shaft so its easy to connect an anode in such a way that it prrtects the prop but not the engine or vice versa. Or you might have a flexible coupling which isolates the prop from the engine electrically. But the more likely answer since you have some resistance rather than infinite resistance is that your wiring is dodgy - a corroded connection somewhere.
pcatterall
Well-Known Member
Cheers birdseye! Figures help! Just done more measurements .... Just for interest 1 m of 0.5mm copper gave me 0.5 ohm a stainless steel tea spoon 1.5 and across my bronze prop 0.7 (yes I am strange but thought I might get a handle on what might be reasonable figures !! )
When I checked annode to prop via the engine and gearbox it was 600 again !!
When I did it again but with the anode inner terminal joined by a cable direct to the prop the reading was 7 ohms. Bearing in mind my other checks I believe the 7 is as good as I can hope for. I will check the only remaining connection (at the engine) otherwise I will be looking for one of those friction contact thingies!
Thanks all again
When I checked annode to prop via the engine and gearbox it was 600 again !!
When I did it again but with the anode inner terminal joined by a cable direct to the prop the reading was 7 ohms. Bearing in mind my other checks I believe the 7 is as good as I can hope for. I will check the only remaining connection (at the engine) otherwise I will be looking for one of those friction contact thingies!
Thanks all again
VicS
Well-Known Member
Actually it's an isolation between engine and gearbox and although it may be so on the MD2010/2040 series it does not apply to all. It is not so, for example, of the Perkins engine used as the MD22. (??)
